Leveling floor for laminate install

I'm trying to level a room in preparation for the installation of laminate flooring. The laminate flooring requires the subfloor to be flat to 1/10" over 4'. The room is 17' x 12' and the walls are supported by the foundation footing. There are two joists running parallel to the 17' wall that are 4' apart. The joists are 4" x 6" beams and the subfloor is 3/4" plywood.
The problem is the two joists are 1/4" lower than the walls. The 1/4" drop is constant along the majority of the room and reduces a bit as the joists approach the end of the room. The two joists are level with each-other.
We have considered the following options to level the floor:
1. Lay 1/4" hardibacker between the joists down the length of the room and screw them in place. Level remainder of the floors to the wall using quicklevel cement.
2. Raise the joists by adding a 1/4" shim at the top of the piers.
We are thinking of going with option 1 because we aren't comfortable performing option 2. We are worried we don't have the skills and we may cause damage. In addition, we don't know where to get a jack to lift the beam the 1/4".
Any suggestions on how to proceed with option 1? We ran a test run with the leveling compound with mixed results. The leveler doesn't appear be 1/4" thick (still drying).
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The leveling compound should get you close enough. 1/10" is a tight tolerance. Are using the thin rolls of foam on the existing floor?If you are out 1/16" in an area, add extra strip of foam. You also can level small areas with building paper, or even 30lb felt paper.The joists are way undersized if your measurement is correct.If the area underneath is a basement or crawlspace, consider adding a beam to cut down the span. mike
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It probably is. Flat does not mean level. Their spec says if you put a 4' straigt edge down in any place, what's the biggest gap. You are closer than you think.

Something is wrong with this picture. Is this really old construction? Is there T&G boards perpendicular to the joists under the plywood? What you are describing is a 44" joist span and that would make just 3/4" plywood feel like a trampoline.

My house is 145 years old; sadly, out of level can be measured in unches in some places.1/4" is *tiny* by old house standards.

The flooring does *not* say level. It says flat. You may have to tweek the flatness with a little floor leveling compound to make the flooring happy, but that's it.
Then again if you want a level floor for some other reason go ahead and build it up.
-Steve
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There are no support boards perpendicular to the joists. The condo is 30 years old and it does bounce a bit.

I understand it doesnt say level but 1/4" over 4 feet is greater than the spec, right? I think this falloff would be fine if it was constant accross the entire floor but it drops that much to the first joist, the floor to the second joist is level, and then raises the 1/4" again to the wall. I just looked at the spec again, it's 2 mm/m (1/12" / 39"1/2).

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My arc geometry is little weak so lets approximate this using triangles. The first 4' (and last, but it's identical) is the only one that we care about. We know that the floor dips 1/4" over this 4'. Let's pretend that the floor drops the entire 1/4" in the first 2' as a straight line, and then is level with the joists from there. If we were to place our imaginary straight edge from the wall to our first joist (4'), the biggest gap would be at the 2' mark, where the floor goes from slanted to level. The "gap" between our straight edge and the floor at that point would be exactly 1/8".
The dip in the floor is probably not a sharp angle like the approximation above. A soft even curve would yield an even smaller gap, bringing you pretty darned close to the flooring spec.
-Steve
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Snipping my own blather....
Better yet, find a really striaght 4' piece of something, set it on the floor and eye-ball it.
-s
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"Stephen M" writes:

What you are trying to create is what the boat builder calls a batten.
Get a piece of aluminum angle, say 3/4"x3/4"x1/16"x96".
Use the back 90 degree corner like a knife edge against the floor.
You will be able to pick up variations that are less than 1/64" with no sweat.
The magic test is when you can't see daylight under the angle any more, you have a fair surface.
SFWIW, it is exactly how my hull was faired out, a 3+ year process.
This surface is far from that critical, but the process is the same.
HTH
Lew
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Thanks everyone for the help with leveling the floor. The leveler seems to have done the trick and now I'm curious about the span of my joists. What is a reasonable way of making the floor more secure? Can I just secure the 3/4" plywood (subfloor) by spanning the joists with 2x4s? Do I need to add additional joists?
-- Craig
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If it was just 3/4" plywood you would KNOW it. It would be very spongy. Maybe therre is a double layer in that sandwich. I doubt your floor is "insecure" but it could be really spongy/springy.
I don't know if this will hlep but is you jump up and down of the floor with all you weight and there is no real movement, you're fine. If it feels spongy, more so between the joists than over them, then 2x4's will help. If the floor is springy (try this over a joist but between piers) you would need bigger joists (sister them) or more piers.
The 44" + only 3/4" plywood span just seems to weird to be true to me. I think there is something else there. Before you install your floor, you could drill a 3/4" hole in the floor in a out of the way spot, stick your finger in there and verify the 3/4" plywood only assertion .
Steve
S

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Good call. I drilled a hole and it's about 1 1/2 " thick. There must be two layers of plywood.
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The levelquick took forever to dry cuz it's been raining. Now that it's dry I can see some crystalline like structures on one pour that we (mistakenly) feathered. The 1/8" thick cement at these edges lifted off the subfloor. Walking on it has also caused a crack of more than 2' along the cement that is ~3/16 thick. Both the cracks and separation occurred parallel to the joists.
Question is, should we lay the laminate over these problems? Re-pour these areas? Start over with a new approach? Help!
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Actually, it's cracking even where not feathered.
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wrote:

How long has the floor been there? Odds are if it's been around a while, and hasn't collapsed, it's fine. You could throw another sheet of 3/4" ply over the works if you're concerned about it. Just lay some construction adhesive down, then drive screws in about every 4 inches or so (screws will help keep the floor from squeaking later on)
Aut inveniam viam aut faciam
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Prometheus wrote:

And they won't pop up. I did nails under vinyl tile. A LOT of nails. Like three and half tons of nails. Six penny ring shanks, like all the books say.
Damn things are popping like crazy. The floor is about ruined after only two years.
I'll use screws next time.
--
Michael McIntyre ---- Silvan < snipped-for-privacy@users.sourceforge.net>
Linux fanatic, and certified Geek; registered Linux user #243621
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